Lightning
Protection for Historic Buildings

Tim Donlon

Figure
1: Eastern Maudit Church, Northamptonshire: An example of the damage
caused below a spire-top air terminal by a side flash. Passing through
masonry from an internal vane rod, the lightning caused the displacement
of solid stonework.

Lightning, an intensely bright spark or streak of light through the
air to ground, has terrified and excited mankind for centuries. Once
considered to be in the realm of the gods, it was only in the past 200
years that the theory of lightning has been transferred to the scientific
realm, initially through the endeavours of Benjamin Franklin (1751).

There
are different types of lightning: cloud to ground, cloud to cloud and
within a single cloud, not to mention such rare variations as ribbon
and ball lightning. On average a cloud to ground strike would be in
the order of 20,000 amperes with a duration of 0.2 seconds, and at its
peak, the power released can be 100 megawatts per metre.

Although relatively few deaths occur (10 per year in the UK(1)), the possibility of a lightning strike to the structure of a building
such as a small church is much greater, at around 1:500(2)per year in the UK. Mechanical effects and damage are primarily caused
by the explosive expansion of air heated to around 30,000ºC, by the
ignition of dust, and by flying debris. Electrical circuits may also
be damaged by the electro-magnetic field generated.

In
a temperate climate lightning is caused by 'frontal' storms which usually
occur as a cold front meets warm air, wedging it upwards thousands of
feet into the atmosphere. Thunderclouds (cumulonimbus) develop as the
moisture condenses and then freezes with altitude. Their development
gives rise to charges building up; in general, ice particles in the
upper part of the clouds are positively charged, while water droplets
lower down contain a negative charge. At the base of the cloud, this
negative charge induces a positive charge at 'ground' level. As the
cloud continues to grow, the charge increases until the voltage difference
between the ground and cloud is so great that the resistance of the
air between the two is broken down and a lightning discharge occurs.
Recent photographs taken from the space shuttle also show discharges
from the top of the thundercloud to the stratosphere; previously such
a phenomenon had not been considered.

The
actual lightning discharge commences with a stepped downward leader
(invisible to the naked eye), negatively charged, searching for an easy
path to earth. This induces a positive charge in the form of an upward
streamer from a structure or object on the ground such as a building
or monument. When the two meet, the potential difference between the
cloud and 'ground' is equalised, causing the bright flash known as the
'return stroke'.

The
effect of a lightning conductor placed appropriately on any building
is to create an 'apparent earth' short circuiting the intense electric
field below a thundercloud. This allows positive ions to be transferred
through the conductor to the atmosphere, as an upward streamer.
At any one time there may be many upward streamers being formed
from a building through various parts of the lightning conductor
system or other non-conductive parts of the building, and the aim
of a lightning conductor designer is to provide suitably placed
conductors based on this knowledge.

An
upward streamer can be formed by all sorts of objects - blades of
grass, trees and manmade structures, notably including ships' masts
and rigging where it produces the phenomenon known as 'St Elmo's
Fire' after the patron saint of Mediterranean sailors.(3)

The
lightning conductor is not an 'attractor' as the movement of positive
ions is governed by the prevailing wind conditions. However, it
does give the current the easiest path to earth, avoiding the transfer
of current through less conductive building materials and the subsequent
structural damage this can cause.

The
points on a structure most vulnerable will be those points nearest
the stepped downward leader on its last step, the length of which
is termed the 'striking distance'. This distance is represented
in the British Standard 6651:1992 (clause 14.3) by a sphere of radius
60m which, in effect, can be rolled around the plan and elevations
of the structure to determine the extent of protection required.
Reducing the radius to less than 60m gives greater protection but
adds to the cost of the installation.

PRINCIPLES
OF PROTECTION

The
provision of a system depends on the type of historic building under
consideration. British Standard 6651:1992 assesses the 'Need for
Protection' against a series of factors concerning the location,
use and construction of a structure. This BS provides an indication
of the relevant factors only and specific calculations should be
carried out if any doubt exists or if the criteria for each index
figure are likely to change.

In
the past British Standards were based on the installation of a single
down conductor to the highest point of a building (British Standard
Code of Practice 326:1965). This approach was designed to create
a 'Zone of Protection' which was formed by striking a 45º line from
the top of the conductor. However, it is now known that this may
not be sufficient, as lightning is known to strike the sides of
tall structures which should not occur if the 45º Zone of Protection
applied to all structures.

Eastern
Maudit Church, in Northamptonshire (Figure 1 above) is one example
of a church spire having a single down conductor which suffered
a 'side flash' through the mortar joints in the solid section of
the spire to the metallic vane rod used to compress the tip of the
spire. This was in spite of the rod being bonded top and bottom
to the down conductor in accordance with British Standard 6651:1992.
The lightning flash appears to have travelled through the open and
weathered joint, similar to the way water moves through minute fissures
via capillary action, and the mechanical force of the strike has
blown a section of the spire stonework away.

A
further example of this phenomenon is illustrated by the unprotected
parapet wall at Lulworth Castle, Dorset which was struck in 1995,
the lightning flash having travelled along the bed joint of the
parapet, dislodging the copings and splitting the masonry below.

Currently
Dr Norman Allen, of UMIST, is researching this phenomenon, which
is currently specifically disregarded in the British Standard 6651:1992
(clause 18.2.2).

The
latest standard recommends that a series of down conductors is installed
to protect the whole building, with towers and spires having a minimum
of two down conductors placed diametrically opposite each other
and horizontal conductors (coronas) vertically at 20m centres. The
remainder of the building should be protected with a series of conductors
interconnected to form a 20m x 10m 'Air Termination' grid, using
where possible elements of the structure, such as metallic gutters,
lead roofs, and so on. Also prominent features such as pinnacles,
crosses, and flêches should have additional air terminals, as they
will form upward streamers under the right conditions.

The
air termination must then be connected to ground with a series of
down conductors spaced around the perimeter of the buildings, one
for every 20m of perimeter (for structures more than 20m high this
is decreased to 10m spaces).

In
order to provide adequate protection in accordance with BS6651:1992
the bonding to electrical and other services, and all extraneous
metal such as flagpoles, bell frames and flues, is essential, to
equalise the potential difference between those items and the lightning
conductor during a strike. This is often ignored in the interest
of expediency and cost.

Once
installed, down conductors and air terminations must be adequately
earthed to ground with a series of electrodes, which may take the
form of either driven rods, plates or mats made from copper. In
some cases a drilled bore hole filled with Bentonite is required
where the underlying strata is rock, and there are special systems
of overlaying conductor tapes. The resistance to earth of each electrode
must not exceed ten times the total number of interconnected electrodes
on a system. (For example, for ten electrodes each one should have
a resistance of less than 100 ohms. This will give an overall resistance
to earth of less than 10 ohms.)

THE
INSTALLATION

Historic
buildings were not constructed with lightning protection in mind,
and the appearance of a new system of air terminals and conductors
can appear intrusive.

The
choice of material is critical in reducing the usual impact of the
system. The cheapest material to specify would be bare aluminium
and PVC clips. These are quite adequate for the purposes of protection
but are ugly and detract from the overall appearance of the structure.
Perhaps the most suitable material for a sympathetic installation
is 8mm solid circular copper conductor with heavy duty cast cable
saddles. Alternately, the conductor can be sheathed with a suitably
coloured PVC to blend with its surroundings. In this way the conductor
appears part of the building rather than contrasting with it.

Figure
2:
All Saints Church, Hereford:
a good example of what can be achieved. This 8mm bare copper
down conductor with heavy duty cable saddles has been positioned
behind the roll and in its shadow line so that it is almost
invisible from all but one aspect.

The
positioning of the conductors is equally crucial to the overall
appearance of the building. While strict adherence to the British
Standard will produce a functional lightning conductor system, by
placing the conductors behind buttresses and out of sight lines,
the whole aspect of the building remains undamaged. This may entail
interpretation of the design criteria of BS6651:1992. However, the
Standard makes allowance for these situations in its guidance notes
and foreword.

Typically,
in a well designed installation, the air termination is hidden behind
parapet walls with short finials, and down conductors are placed
behind pinnacles and in returns of buttresses or other key features.
They should always be straightened and installed with a string line,
following the lines of the building. On rubble or pitch faced stonework
they should not be dressed into each crack and contour of the stone
as the conductor then looks poorly installed in profile.

It
is always prudent to use the features of the building to mask the
visual effects of a conductor. For instance, it is prudent to shadow
a strong feature on a spire or tower, following a stone quoin or roll
as the human eye will focus on the more prominent feature and disregard
a conductor placed next to it (Figure 2).

The
down conductor must have a facility for testing the earth electrode
below: a purpose made test clamp should be positioned in each down
conductor at a uniform height to allow access for testing but not
to encourage tampering. Electrodes are generally driven into the
ground in multiples of 1.2m lengths; it is therefore essential to
check for underground hazards, such as services and archaeological
artefacts, before deciding on their positions.

Once
installed, the lightning conductor system should be commissioned
and maintained with an inspection and a test of each electrode carried
out by an approved contractor annually and immediately following
a lightning strike. There is little sense in having a system carefully
designed and installed which is not adequately maintained.

The
installation of a lightning conductor to an historic building also
requires the selection of a qualified contractor, who is capable
of understanding the importance and delicacy of the various features
and the nature of the construction to which the system is being
installed. The National Federation of Master Steeplejacks and Lightning
Conductor Engineers aims to provide a standard of quality and service
through its members who have achieved the necessary level of competence
in all aspects of their work, emphasising quality of training and
professionalism combined with 'Safety through Training'. More damage
may occur to the fabric of the structure through selecting the contractor
on price alone than may be caused by a lightning strike, so all
factors available should be considered before deciding to proceed.

THE
IMPORTANCE OF ADEQUATE PROTECTION

The
provision of a lightning conductor system will not prevent the occurrence
of a lightning strike. The purpose of the installation is to direct
the current discharged from the strike to earth safely, protecting
the structure and its occupants from the effects of the strike.

At
one church where there was a lightning conductor system (albeit
not up to current standards) the bell ringers of Yoxall decided
to continue to ring during a thunderstorm. They later described
how 'the air was full of bright butterflies, the carpet was
swimming with a haze of electric blue cobwebs and the pins and needles
lasted for hours'. Their decision to stay turned out to be
a wise choice since 'had we decided to call it a night, we
would most certainly have been below the falling pinnacle'
(The Ringing World, no 4376, March 10th, 1995).

British Standard 6651:1992 - Protection of Structures against
Lightning, British Standards Institute, 1992

This
article is reproduced from The Building Conservation Directory, 1997

Author

TIM DONLON BSc,
MCIOB, works for Church Conservatoin Limited of Nottingham as Technical
Contracts Manager. The Company, which is a member company of the National
Federation of Master Steeplejacks and Lightning Conductor Engineers,
specialises in the care and conservation of historic properties.